Electric arc welding involves the passing of an AC or DC current between a metal electrode and a workpiece where the metal electrode is normally a cored metal wire or solid metal wire. A power source is used to create a given current pattern and/or polarity between the advancing electrode wire and workpiece so that the arc will melt the end of the advancing welding wire and deposit the molten metal on the workpiece. Although various converter technologies are used for power sources, the most effective is an inverter based power source where a switching network includes switches operated at high frequency to create the desired waveform or current level for the welding process. An inverter type power source is discussed in Blankenship U.S. Pat. No. 5,278,390 where the inverter is controlled by “waveform control technology” pioneered by The Lincoln Electric Company of Cleveland, Ohio. The actual waveform is generated by a series of short pulses created at a frequency generally above 18 kHz and the group of short pulses has a profile controlled by a waveform generator. In accordance with standard power source technology, the input signal to the inverter stage of the power source is rectified current from a sine wave power supply. An appropriate power factor correcting converter is common practice and is either a part of the inverter switching network itself, as shown in Kooken U.S. Pat. No. 5,991,169, or is located before the inverter stage, as shown in Church U.S. Pat. No. 6,177,645. Indeed, a power source with a power factor correcting converter or stage has been known in the welding art for many years. Another power source employing an input power factor correcting converter in the form of a boost converter is shown in Church U.S. Pat. No. 6,504,132. The two patents by Church and the patent by Kooken are incorporated by reference herein as background information. In both Kooken U.S. Pat. No. 5,991,169 and Church U.S. Pat. No. 6,504,132 the actual welding current is regulated by an output chopper or buck converter and isolation is obtained by a transformer either in the output of the inverter stage or in the output of the input boost converter. These various topologies for power sources are common knowledge in arc welding technology. In these prior art patents, the actual welding current, voltage or power is regulated in or before the output stage of the power source, which output stage is either an inverter or a chopper. Neither the inverter, nor the chopper is unregulated to produce a fixed, lower voltage DC bus for driving a regulated welding stage.
Isolation of the welding operation is a characteristic of most power supplies for welding. The term “welding” includes “plasma cutting.” In Vogel U.S. Pat. No. 5,991,180, a preregulator using a boost converter is directed to a converter which is disclosed as a chopper having an output isolation transformer located after welding regulation and directly driving the welding operation. In this power source, the chopper network is controlled to create the desired regulated output welding current and isolation is provided in the output stage. In a like manner, Thommes U.S. Pat. No. 5,601,741 discloses a boost converter for driving a pulse width modulated controlled inverter providing the regulated output signal to the actual welding operation. In both Vogel and Thommes, the second stage is regulated to direct the power factor controlled current from a preregulator into a welding operation. Welding regulation is in the second stage and is normally driven by a pulse width modulator control circuit. Both Vogel and Thommes are incorporated by reference herein as background technology. In Moriguchi U.S. Pat. No. 6,278,080 an inverter type power source is regulated to control the desired welding current. Isolation is obtained by a transformer between the controlled second stage inverter and the welding output which is disclosed as a DC welding operation. A similar power source is shown in Moriguchi U.S. Pat. No. 5,926,381 and Moriguchi U.S. Pat. No. 6,069,811 wherein the isolation of the control current from the inverter stage is at the output of the inverter and directly drives the welding operation. Moriguchi U.S. Pat. No. 5,926,381 discloses the common arrangement for using the voltage at the output of the first stage boost converter to provide the controller voltage for either the regulated inverter stage or the boost converter itself. The three Moriguchi patents are incorporated by reference herein as background information showing the prior art power source where a regulated inverter is driven by an input boost converter or a DC output of a rectifier to produce a controlled welding current directed to an output transformer used for isolation. The secondary AC signal of the isolation transformer is directly used for the welding operation. There is no third stage topology as used in the novel power source of the invention.
Turning now to non-welding technology, an aspect of the invention is the use of a synchronous rectifier device at the output of a DC/DC second stage converter. Synchronous rectifiers are common practice and one such rectifier is illustrated in Boylan U.S. Pat. No. 6,618,274. Calkin U.S. Pat. No. 3,737,755, discloses a DC/DC converter for low power use where a fixed regulated current is directed to a non-regulated inverter to provide a non variable output DC signal. Any control of the non-regulated inverter is at the input side of the inverter so that the input DC signal is the only parameter that can be regulated to control the fixed output DC signal of the inverter. This is a topography that requires a control of the signal to the inverter so that the inverter provides a controlled fixed output signal. The non-welding general background technology in Boylan and Calkin is incorporated by reference herein to show a synchronous rectifier and a version of a non-regulated inverter where any regulation is performed before the inverter by controlling the level of the input DC signal. Neither of these patents relate to a power source for welding and are only incorporated by reference as general technical concepts, such as synchronous rectifier devices and unregulated inverters. A non-welding two stage AC to DC converter is shown in Smolenski U.S. Pat. No. 5,019,952 for imparting minimum harmonic distortion to the current flowing into the converter. The load is not variable and does not require regulation as demanded in a welding operation. This patent is incorporated by reference to show general technology not related in any way to the demands of a power source for electric arc welding.
These patents constitute the background information relating to a power source that must be regulated by a welding operation where such regulation is by a feedback loop of average current, average voltage, and power of the actual welding operation. Fixed load power sources are not relevant to the invention, except as general technical information.
In the past, an inverter in a power source outputted a welding current regulated by a parameter in the welding operation, such as current, voltage or power. This inverter was normally controlled by a pulse width modulator wherein the duty cycle of the switches operated at high frequency was controlled by the feedback from the welding operation so that the duty cycle was adjusted in a range substantially less than 100%. This type of PWM controlled inverter is referred to as a regulated single stage inverter. Such inverter formed the output of the power source and was the last stage of the power source. Lower duty cycles resulted in higher primary currents and more losses. The efficiency of the inverter varied according to the duty cycle adjustment caused by the requirement of regulating the output of the single stage inverter to create an output signal suitable for welding. Using a power source where the final stage is a regulated single stage inverter resulted in heat losses, lower efficiency, high cost and increased component size. For these reasons, some welding source manufacturers have marketed power sources as being better than an inverter power source because they do not use inverters with the resulting high cost and other difficulties. An inverter stage which had the dual function of isolating the output and regulating the current for the purposes of creating a current suitable for welding was to be avoided. See Hoverson U.S. Pat. No. 6,723,957, incorporated by reference herein as background.